For the objectives of saving energy and preventing global warming, capacitors or secondary batteries are used in various fields, particularly in the automobile industry. Due to adoption of electric energy, development of technologies utilizing these is being accelerated.
At present, a double electric layer capacitor is used as a backup of a memory of a circuit applying a low voltage, and compared with a secondary battery, has relatively higher input and output reliability.
Therefore, in recent years, double electric layer capacitors are used for natural-energy-based power generation such as photovoltaic or wind power generation or construction machinery and voltage sag power supplies, renewable power supplies of trolleys, and the like, Uses for automobiles are also researched. Characteristics and cost do not meet requirements yet, and up to now, uses in this field have yet not been realized. However, at present, double electric layer capacitors are used for electronic control brake systems, and researches on uses for starting energy supply, brake control, power assisting and the like of standby power supplies of electrical components and idling stop systems of automobiles are being made.
A double electric layer capacitor is structurally formed by positive and negative electrode parts, an electrolyte solution and a partition plate for preventing short-circuit of the opposite positive and negative electrode parts. Polarizable electrodes (mainly activated carbon at present), a binder for retaining the active carbon and a conductive agent (mainly carbon microparticles or fine fibers) are mixed and kneaded for coating on an aluminum foil (with a thickness of about 20 μm) which is taken as a current collector, and multiple layers are formed by coating, thereby forming the electrode parts. For example, such a double electric layer capacitor is disclosed in Patent Document 1.
Electrolyte ions move in a solution, and are adsorbed or desorbed on surfaces of micropores of the activated carbon, thereby charging the double electric layer capacitor. Double electric layers are formed on a contact interface of activated carbon powder and the electrolyte solution.
Here, the activated carbon usually has a particle size of, for example, about 4˜8 μm and a specific surface area of, for example, 1,600˜2,500 m3/g. The electrolyte solution contains cations, anions and a solvent, tetraethylammonium salt is used as the cations, tetrafluoroboric acid ions are used as the anions, and propylene carbonate, ethylene carbonate or the like is used as the solvent.
On the other aspect, a lithium ion secondary battery is mainly formed by a positive electrode, an negative electrode and a partition plate. For example, as shown in FIG. 8, under a normal condition, the positive electrode is formed by coating a 20 μm-thick aluminum foil taken as a current collector with a 100 μm-thick substance obtained by blending active substance powder (usually lithium cobaltate), a conductive agent taken as an additive and a binder, the negative electrode is formed by coating a copper foil taken as a current collector with a carbon material, and these are partitioned by the partition plate such as polyethylene and soaked in an electrolyte solution, thereby forming the lithium ion secondary battery. Such a lithium ion secondary battery is disclosed in Patent Document 2.
Charging and discharging are implemented by movement of lithium ions between the positive electrode and the negative electrode. During charging, the lithium ions move from the positive electrode to the negative electrode, and when the lithium ions of the positive electrode disappear or no lithium ions may be stored in the negative electrode anymore, charging is ended. Discharging is reverse to charging.